f2fs: Optimize f2fs_truncate_data_blocks_range()

Function f2fs_invalidate_blocks() can process consecutive
blocks at a time, so f2fs_truncate_data_blocks_range() is
optimized to use the new functionality of
f2fs_invalidate_blocks().

Add two variables @blkstart and @blklen, @blkstart records
the first address of the consecutive blocks, and @blkstart
records the number of consecutive blocks.

Signed-off-by: Yi Sun <yi.sun@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>